Conventional pumps have a rising discharge characteristic directly responsive to pump speed. Pumps of that kind may be vane or gear pumps. Such pumps may be used for applicant's purpose in hydraulic installations where it is not material that increase in speed is not effective to reduce pump discharge, i.e., increase in discharge with increase in speed may be desired in some circumstances. In such systems pressure is limited by a valve to permit excess discharge to bypass the oil tank.
Radial piston pumps are well known as exemplified in German Pat. No. 29 02 240 and are used for the advantage of an operating characteristic wherein outlet discharge is reduced inversely as pump speed rises, an effect caused by throttling the suction feed to the pump. However, the regulation point of the outlet flow is at a relatively low speed of about 1200 RPMs and then remains almost constant.
Where such a pump is used in booster steering systems of motor vehicles a decreasing pump output characteristic is desirable as the speed of the vehicle increases. Thus, the pump which is driven by engine speed has an increase in speed and at such times a reduced discharge of pressure oil is desired. By effecting such inverse response a vehicle traveling at high speeds at which no large steering deflections are needed, booster steering is improved in driving characteristic when the boost steering system is in use since the driver has a better feel of contact with the road bed, a very desirable condition.
It has heretofore been known from German patent OS No. 29 06 047 (FIG. 3) to provide an electrical throttling valve in the discharge line of a booster steering pump which valve automatically effects throttling of discharge responsive to an electrical signal corresponding to travel speed of the vehicle. Accordingly, an electrical control throttle valve controls pressure oil to a consumer device such as a power cylinder in such a way that in case of slow travel the full pump discharge is available. However, in the case of fast travel the pump discharge is greatly decreased whereby excess oil flows back into the oil tank. A throttle arrangement control of the pump as described is expensive and operates in combination with a pump which has no regulation of suction feed and particularly at higher speeds has a relatively high leakage current.